1. Field of the Invention
The invention relates to wrapping a load with packaging material, and, more particularly, to stretch wrapping.
2. Description of the Related Art
Various packaging techniques have been used to build a load of unit products and subsequently wrap them for transportation, storage, containment and stabilization, protection and waterproofing. One system uses stretch wrapping machines to stretch, dispense and wrap stretch packaging material around a load. Stretch wrapping can be performed as an inline, automated packaging technique which dispenses and wraps packaging material in a stretch condition around a load on a pallet to cover and contain the load. Pallet stretch wrapping, whether accomplished by a turntable, rotating arm, or rotating ring typically covers the four vertical sides of the load with a stretchable film such as polyethylene film. In each of these arrangements, relative rotation is provided between the load and the packaging material dispenser to wrap packaging material about the sides of the load.
Wrapping packaging material about the sides of the loads typically unitizes and stabilizes the load. However, such side wrapping generally does not cover the top of the load or secure the load to the pallet in the manner which would promote increased stability. Because of the structure of a typical stretch wrap apparatus, it is difficult to wrap packaging material about the top and bottom of the load to secure the load to the pallet for stability. Previous attempts to wrap packaging material about the top and bottom of a load include holding a palletized load on the tines of a forklift truck and placing the load and tines supporting the load within a wrapping mechanism to be wrapped. This method requires the driver of the forklift truck to carefully control the timing and position of the truck and the wrapping machinery revolving around the load and tines of the forklift truck to wrap packaging material about the top and bottom of tile load to avoid undesirable interference between the truck, the load and the wrapping machinery during wrapping. Alternatively, the top and bottom of the load have been wrapped by conveying a load through a wrapping ring on a dual conveying mechanism such that after wrapping, the load is wrapped to the conveyor and the dual conveyor must move the load and the packaging material away from the wrapping area together. Such devices are expensive, requiring structure to keep the load and the packaging material moving at the same speed along the conveyor, preventing the packaging material from being caught on or torn, and arrangements to get electrical power to the rotating portion of the ring for controlling a dispenser mounted on the ring. In another alternative, a load is positioned and wrapped on a cantilevered load support having a free end in the wrapping area such that a cantilevered packaging material dispenser is rotated about the load on the cantilevered load support below the free end of the cantilevered load support. Thus, the load is wrapped to the cantilevered load support and then the load must be pushed off or carried off of the load support by the following load or taken off with a conveyor. However, there is a high degree of friction involved with such movement off of the load support which may cause disorientation of the load or the film.
Over the past fifteen years many machine developers have struggled with conveying packaging material off the base of the load support and off of conveying systems. The problem is complicated by characteristics of stretched packaging material being pulled transversely from the direction that has been stretched wrapped around a load. The wrapped force may range up to 800 pounds for a 48-inch long load.
Additionally, the packaging material is typically spirally wrapped and made up of up to 40 individual wraps. Due to the nature of the spiral, some packaging materials develop ropes along their edges. The packaging material is designed to bond to itself and therefore is quite tacky. These characteristics make it difficult to slide the packaging material over any fixed surface where significant forces are incurred. Several approaches have been disclosed to drive the packaging material on a conveying means parallel to the direction of the travel of the load. These include patents issued to Lantech Inc. and to Keip Machine Co. These systems depend on relatively expensive and complex drive mechanisms to drive the packaging material independently of the force of the load.
Due to the expensive nature of the independent drives required to drive the packaging material and the load, other attempts have been made to use non-powered mechanisms to carry the packaging material. Such attempts include the use of rollers, belts, chains, low friction coatings, air bearings, slider bars, screws, reciprocating feet, and air jets for a non-powered packaging material carrier. Each of these has suffered difficulty in robustly allowing the transverse movement of the load to slide the packaging material off the load support conveyor or platform. High drag force can distort the load, split the packaging material or cause the load drive conveyor to slip. Particular problems with attempts to use rollers and wheels include offsetting the wheels which allows them to catch the loose packaging material, and allowing ropes of packaging material to become caught between the wheels and thus lock the wheels, preventing the packaging material from moving along the rollers. In a further attempt to create a non-powered device, side bars were added to the rollers to carry some of the force of the packaging material and prevent jamming of the rollers. However, the friction created between the packaging material and the bars was too great, preventing easy movement of the packaging material and causing tearing of the packaging material and sticking between the packaging material and side bars.
In light of the drawbacks associated with providing expensive powered conveyors which move the packaging material and the load at the same speed and the friction problems associated with simply pushing the load off of a load wrapping surface, there is a need to wrap the top and bottom of the load with packaging material in the simple, reliable and inexpensive manner which will also allow for the removal of the load from the wrapping surface without tearing, friction or expensive mechanisms to do so. The present invention solves the problem of the delicate balance between protecting the wheels from locking up and prevention of a high friction contact.